Electronic devices are typically housed in a "package". The package is designed to provide the device protection from environmental factors, such as moisture. It is also designed for convenient electrical interconnection and for conduction of heat generated by the circuit to the outside of the package, where it may be removed by various means. There are many different types of packages commercially available to the manufacturers of electronic circuits. To some extent the wide use of certain types has given them the reputation as a "standard" design geometry identified by a designation recognized in the industry through validation by trade organizations, such as the IEEE (Institute of Electronic and Electrical Engineers).
Heat removal from circuit devices in packages has become increasingly challenging as their power densities have increased. For example, large scale integrated circuits include ever larger numbers of active elements and operate at ever higher speeds, which requires more power. Power amplifiers have been reduced in size, to that they have higher power densities. More effective heat removal has been accomplished by the incorporation into the package design of materials which have a high heat conductivity, such as copper. Usually, the copper is in the form of a substrate of the package, and the circuit device is mounted directly to it with a suitable bonding agent. In some cases the circuit device cannot be mounted directly on an electrically conducting member because spurious noise in its signal would arise due to capacitive coupling to the substrate. In that case either the conductive substrate must be covered with an electrically insulating layer before the circuit device is attached, or the substrate must itself be made of an electrically insulating, thermally conductive material. Packages of this general type are described, for example, in U.S. Pat. Nos. 5,065,281 and 5,109,268.
While there are a number of materials, such as aluminum nitride, beryllium oxide, aluminum oxide, and silicon nitride, which are electrically insulating and also effective conductors of heat, by far the most outstanding material in this category is diamond. In particular, diamond film can be made by CVD (chemical vapor deposition) in wafer form suitable for use as a substrate material. It has a very high electrical resistivity and is about three to five times as effective in conducting heat as is even copper, the best of the metals in this regard.
The incorporation of diamond film into a package for heat removal involves a number of difficulties. Among these is that the CTE (coefficient of thermal expansion) of diamond is much lower than that of the metals typically used in packages. This can lead to extreme stresses and eventual mechanical failure of the package itself or of the circuit device housed by it. Attempts to alleviate mismatching of the CTE often lead to multi-layered structures in which the heat conductivity of the diamond material is not used to full advantage due to the thermal impedances introduced by intervening interfaces or the external geometry of the package becomes altered, so that it can no longer be used in the same way for all applications. Such a change in geometry is often unacceptable to the circuit device manufacturers who use the package because it requires specialized assembly equipment and processes.
There is a need for a circuit package which successfully incorporates diamond film into a circuit device package for heat removal while avoiding damaging thermal mismatch with other materials attached to it and minimizing any detraction in its performance for removing heat from the circuit device to the outside of the package.